1. Field of the Invention
The present invention relates to a deposition mask and a method of preparing the same, and more specifically, it relates to a deposition mask employed for depositing a deposit material such as an organic EL (electroluminescence) film on a substrate and a method of preparing the same.
2. Description of the Prior Art
A deposition mask employed for depositing a deposit material on a substrate is known in general. Such a deposition mask is employed for depositing an organic EL film in the process of preparing an organic EL display for color display for forming the organic EL film serving as an emission layer, for example. FIG. 22 is a sectional view showing a conventional metal deposition mask employed for depositing an organic EL film. FIG. 23 is a model diagram showing the process of depositing the organic EL mask through the conventional metal deposition mask shown in FIG. 22.
Referring to FIG. 22, a plurality of mask openings 102 each having a vertical opening section are provided on a metal mask substrate 101 in the conventional deposition mask. The mask openings 102 are formed by etching or mechanically working the mask substrate 101.
In order to perform deposition through the metal mask substrate 101, the mask substrate 101 is first set on a position separated from a deposition side surface of a target substrate 105 at a prescribed interval, as shown in FIG. 23. Deposit particles 104 are scattered from a deposition source 103 toward the target substrate 105. Thus, the deposit particles 104 are deposited on the target substrate 105 through each mask opening 102 of the mask substrate 101, to form a deposit 108.
In general, the deposit particles 104 scattered from the deposition source 103 for deposition through the mask substrate 101 exhibit directivity as shown in FIG. 23. In this case, each mask opening 102 of the mask substrate 101 has a vertical section and hence a shadow 106 is defined on the deposit 108 formed on the target substrate 105 by an end of the mask opening 102 closer to the deposition source 103, to result in an nonuniform thickness of the deposit 108. Particularly when the mask substrate 101 has a large thickness, the length of the shadow 106 is so increased that the thickness of the deposit 108 is reduced on an end thereof. Thus, nonuniformity of the thickness of the deposit 108 is disadvantageously increased.
In order to solve the aforementioned problem, the thickness of the mask substrate 101 may be reduced. FIG. 24 is a model diagram for illustrating a problem in a conventional metal mask substrate 101 having a small thickness. Referring to FIG. 24, the metal mask substrate 101 having a small thickness is reduced in mechanical strength, to be readily deflected. In particular, the conventional metal mask substrate 101 has such large specific gravity that the same is readily deflected when reduced in thickness. When the mask substrate 101 is deflected, the distance between the mask substrate 101 and a target substrate 105 is rendered nonuniform to result in such a new problem that patterns of deposits 108 different from mask patterns 107 are formed.
As hereinabove described, the shadow 106 of the conventional metal mask substrate 101 is disadvantageously increased to result in a nonuniform thickness of each deposit 108. When the thickness of the mask substrate 101 is reduced for solving this problem, patterns of the deposits 108 different from the mask patterns 107 are disadvantageously formed. Thus, it is difficult to obtain desired patterns of the deposits 108 in general.
As shown in FIG. 25, there has been proposed a structure forming each mask opening 102a of a metal mask substrate 101 in a tapered shape having an opening width increased toward a deposition source 103. This structure is disclosed in Japanese Patent Laying-Open No. 10-298738 (1998) or 10-319870 (1998), for example. When the mask opening 102a has a tapered shape with an opening width increased toward the deposition source 103, a shadow 106 is reduced regardless of the thickness of the mask substrate 101 so that the length of a part of a deposit 108 having a small thickness on its end can be reduced. Thus, nonuniformity of the thickness of the deposit 108 can be relaxed. The aforementioned gazette discloses that the thickness of the metal mask substrate 101 having the tapered mask opening 102a is 200 μm to 500 μm.
The relation between displacement of a deposit pattern and a cone angle in the conventional deposition mask shown in FIG. 25 is now described with reference to FIGS. 26 and 27. In the following description, it is assumed that deposit particles 104 passing through each mask opening 102a straightly advance to a target substrate 105. In other words, it is assumed that the amount of inwardly deviating deposit particles 104 is extremely small and ignorable.
Referring to FIG. 26, the relation between displacement of a desired deposit pattern and the cone angle of the deposition mask is described with reference to the deposit particles 104 scattered with directivity. Symbols in FIG. 26 denote the following factors:                L: a perpendicular connecting the center of the deposition source (not shown) with the target substrate 105        r: the distance between the perpendicular L and the mask opening 102a         t: the thickness of the mask substrate 101        g: the distance between the target substrate 105 and the mask substrate 101        h: the distance between the target substrate 105 and the deposition source        s: the opening width of the opening 102a of the mask substrate 101        θ: the angle formed by the perpendicular L and the direction of the scattered deposit particles 104        θ0: the cone angle of the mask opening 102a         a0: the width of pattern displacement on the inner periphery of the mask opening 102 (the distance between the position where the deposit particles 104 passing through a portion close to the inner periphery of the mask opening 102a reach the target substrate 105 and the toe of a perpendicular connecting the inner periphery of the mask opening 102a with the target substrate 105)        b0: the width of pattern displacement on the outer periphery of the mask opening 102a         
The amount Δw of increase/decrease of the width of the actual deposit pattern with respect to the opening width s of the mask opening 102a and the average horizontal displacement ΔD between the desired pattern (the opening of the mask opening 102a) and the actual deposit pattern are expressed as follows:Δw=b0−a0ΔD=(a0+b0)/2
Referring to FIG. 26, the cone angle θ0 of the mask opening 102a is greater than or equal to the angle θ (θ0≧θ) formed by the perpendicular L and the direction of the scattered deposit particles 104. In this case, influence by a shadow resulting from the thickness t of the mask substrate 101 can be ignored in the deposit pattern. Therefore, the amount Δw of increase/decrease of the deposit pattern is expressed as follows:Δw=0  (1)
The displacement ΔD is not influenced by the thickness t of the mask substrate 101 either, but is expressed as follows:ΔD=a0=b0=g×tan θ  (2)
As clearly understood from FIG. 26, tan θ is expressed as r(h−g). In this case, the distance g is sufficiently smaller than the distance h in general, and hence tan θ is approximated as follows:tan θ=r/h  (3)
Hence, the above equation (2) is transformed into the following equation (4):ΔD=g×r/h  (4)
In this case, the displacement of the deposit pattern from the mask pattern is expressed by the above equations (1) and (4).
Referring to FIG. 27, the cone angle θ0 of the mask opening 102a is less than the angle θ (θ0<θ) formed by the perpendicular L and the direction of the scattered deposit particles 104. In this case, the width of pattern displacement on the inner peripheral side is decided by the surface of the mask substrate 101 closer to the deposition source. In this case (θ0<θ), the amount Δw of increase/decrease of the width of the actual deposit pattern with respect to the opening width s and the average horizontal displacement ΔD between the desired pattern (the opening of the mask substrate 101) and the actual deposit pattern are increased as compared with those show in FIG. 26.
Thus, it is important that the cone angle of the mask opening 102a of the mask substrate 101 is greater than the angle of the scattered deposit particles 104.
The relation between the thickness of the mask substrate 101 and the pattern interval is now described with reference to FIGS. 28 and 29. Symbols in FIGS. 28 and 29 denote the following factors:                t: the thickness of the mask substrate 101        s: the width of the mask opening 102a         d: the width of a non-opening part (the distance between the openings)        θ0: the cone angle of the mask opening 102a         
While FIGS. 28 and 29 illustrate the width s as less than the width d (s<d) for the purpose of convenience, the width s is greater than the width d (s>d) in practice. In an organic EL display, a deposit is deposited on the target substrate 105 to form an emission part. In the non-opening part, no deposit is deposited on the target substrate 105 but a non-emission part is defined. In order to improve the screen of the display in brightness as well as in definition, the width d of the non-opening part defining the non-emission part is preferably minimized. The width d of the non-opening part is minimized when the non-opening part of the mask substrate 101 has an inverse-triangular section, as shown in FIG. 29. The minimum value of the width d of the non-opening part is expressed as follows:d=2t×tan θ0  (5)
As clearly understood from the above equation (5), the minimum value of the width d of the non-opening part of the mask substrate 101 is decided by the thickness t of the mask substrate 101. In other words, the width d of the non-opening part of the mask substrate 101 can be reduced by reducing the thickness t of the mask substrate 101. When the width d of the non-opening part of the mask substrate 101 can be reduced, the screen of the display can be improved in brightness as well as in definition.
Japanese Patent Laying-Open No. 10-298738 disclosing the aforementioned conventional tapered mask opening 102a describes the following values as the conditions for a deposition method employing the metal mask substrate 101:                t=0.2 mm (t: the thickness of the mask substrate 101)        g=0.01 mm (g: the distance between the target substrate 105 and the mask substrate 101)        h=400 mm (h: the distance between the target substrate 105 and the deposition source)        r: 100 mm (r: the distance between the perpendicular L and the mask opening 102a)        
From the above equation (3), the effective cone angle of the mask opening 102a is expressed as follows:tan θ0=r/h=0.25Hence,θ0=14°
The thickness t of the mask substrate 101 is 200 μm, and hence the minimum value of the width d of the non-opening part under this condition is expressed as follows from the above equation (5):d=2t×tan θ0=0.1 mm=100 μm
In order to enable the organic EL display to improve the screen in brightness as well as in definition, the width d of the non-opening part must be not more than 50 μm. Assuming that the cone angle θ0 is left intact, the thickness t of the mask substrate 101 must satisfy the following equation from the above equation (5), in order to set the width d of the non-opening part to 50 μm:t=d/tan θ0/2=100 μm
In other words, the thickness t of the mask substrate 101 must be not more than 100 μm, in order to set the width d of the non-opening part to not more than 50 μm. However, the conventional metal mask substrate 101 is extremely deflected by its own weight if the thickness thereof is reduced due to the large specific gravity as described above, and hence no desired pattern can be obtained. The conventional metal mask substrate 101 generally has a thickness of 200 μm to 500 μm, as described in Japanese Patent Laying-Open No. 10-298738 or 10-319870. In other words, it is generally difficult to form the metal mask substrate 101 in a thickness smaller than 200 μm. Therefore, the width d of the non-opening part generally exceeds 100 μm, and it is difficult to improve the screen of the organic EL display in brightness as well as in definition.
FIG. 30 shows deformation (deflection) Z of the mask substrate 101 caused by the largest factor of gravity (own weight). Referring to FIG. 30, the deformation Z of the mask substrate 101 caused by gravity is proportional to the specific gravity ρ, inversely proportional to the Young's modulus E, and inversely proportional to the cube of the thickness t as follows:Z∝ρ/E/t3  (6)
When the thickness t is reduced, therefore, a material having small specific gravity and a large Young's modulus is suitably employed. However, the conventional metal mask substrate 101 having large specific gravity does not satisfy these conditions. When the thickness t of the mask substrate 101 is reduced, a working technique of providing a tapered opening is required. In the conventional metal mask substrate 101, however, it is difficult to precisely work a tapered mask opening when the thickness t is reduced.